To some people, the San Francisco Bay area of the USA is just the access to the Pacific Ocean between the state of California’s metropolitan areas of San Jose, San Francisco and Oakland.
Actually, it is a shallow estuary connecting numerous smaller bays, and forming a sensitive aquatic environment— an essential life-sustaining interface where the ocean greets 40% of California’s watershed. The Sacramento and San Joaquin rivers both empty there, making the bay a significant ecological habitat.
But it hasn’t always been treated this way. Starting in the Gold Rush of 1849, as people settled into the area, they diked, dredged and filled the bay’s waters to improve land for agriculture and housing. In total, more than 85% of bay wetlands were drained. By the 1970s, awareness of the bay’s ecological peril triggered legislation mandating both environmental reform and region-wide restoration projects. Some of the wildlife, identified as uniquely dependent on the bay habitat, was listed as endangered species.
So it’s not hard to imagine why words like “explosives” and “implosion” generated high levels of concern when they were used to discuss how the 80-year-old San Francisco-Oakland Bay Bridge’s 21 piers should be demolished in 2015.
Yet this is exactly what was proposed.
In theory, an explosion less than 10 seconds long should have significantly less effect on the bay wildlife and water quality than a four-year-long, piece-by-piece removal of the structures from within a fabricated coffer dam drained as a worksite, but only if a curtain of bubbles could protect the bay wildlife from the shock and noise.
The Pier E3 implosion conducted in November was designed as a practical demonstration of the curtain bubble strategy. It was successfully completed on Pier E3. The largest of the piers, E3 is the equivalent of a five-storey concrete building.
The demonstration was designed to help determine the best choice for the remaining piers. Numerous local, state and federal agencies reviewed it first, namely the Department of Fish and Wildlife, National Marine Fisheries, National Oceanographic and Atmospheric Administration, Bay Area Conservation and Development Commission, and US Army Corps of Engineers.
The early November date was chosen as having the least potential impact on bay wildlife, at a time when bird nesting and spawning and migrations of salmon, trout and herring are at a lull. Nevertheless, multiple wildlife experts designed and manned wildlife and water quality monitoring systems throughout the test.
Leah Robinson-Leach, Bay Bridge spokesperson from the California Department of Transportation, said, “This pier implosion was the culmination of years of collaborative preparation and planning. The combination of science, technology and engineering with environmental stewardship is historical.”
General contractor Kiewit/Manson was in charge of the operation, using 1,600 cfm oil-free compressors from Atlas Copco Rental to supply air at full capacity to the blast-buffering bubble curtain.
Blake Gearhart, vice President of operations west for Atlas Copco Rental, said that the air compressors are California Air Resource Board (CARB) permitted.
“Kiewit/Manson is a socially responsible company and they needed diesel-powered air compressors. We were an obvious choice because our compressors are CARB permitted.”
Brian Ford, Atlas Copco Rental area sales manager, said assembling a fleet of so many compressors for a job is not a small undertaking.
“We were confident we could provide what they needed,” he said.
“We have the largest fleet in North America of 100% oil-free compressors that are both diesel-powered and CARB-permitted.”
Brian added that the high-volume Atlas Copco Rental models were particularly suited to an environmentally sensitive project because they introduce 100% oil-free air into the water. As an added measure of safety, each compressor’s base provides fluid containment.
The 16 compressors sat eight to a barge, one barge anchored to either side of the pier’s foundation. The compressors were connected to a single steel framework consisting of three perforated 3 in PVC pipes. The frames were lowered to the bay floor 50 ft below the barges.
On the day of the demonstration, all eyes were on the pier. It was streamed live online and given live coverage on television. Bay Bridge traffic was momentarily halted for the blast, a precaution against drivers becoming startled by the sudden noise. The Transbay Tube, a BART (Bay Area Rapid Transit) light rail service running beneath the bay, was ceased for in-tunnel monitoring of the blast.
For six seconds, a sequential series of blasts from 600 explosive charges rumbled through the structure, fracturing the reinforced concrete, which collapsed into the pier’s own honeycomb-like base below the bay floor. It left nothing to retrieve but the blast covers floating in the foam, an attenuation mat of large, connected planks placed over top of pier, had been over the structure as an added precaution against potential flyrock – the rock propelled beyond the blast area by the force of an explosion.
Maintaining the curtain
The compressors maintained the curtain. One simple test had been to place fish in cages at various distances from the blast to measure its effect on them, and none of the fish were affected by the implosion. Analysis of free-swimming fish captured in trawling nets concluded there had been no harm.
Within a five-mile observation area, scientists specialising in identification and behaviour of aquatic mammals stationed themselves to watch for signs of impact to the bay’s sea lions, harbour seals, harbour porpoises, elephant seals or whales.
CalTrans – the California Department of Transportation – reported: “Initial studies including three days of round-the-clock observation after the blast have determined no injuries or deaths to aquatic wildlife.”
The result has surpassed initial expectations. Bay Bridge chief engineer Brian Maroney said: “This method has proven itself to be the most cost-effective and environmentally friendly.”
Brian extended his gratitude to the collaborative force that had made the demonstration such a success. “I just cannot offer enough appreciation to all the resource agencies and particularly San Francisco Bay Conservation and Development Commission, for supporting this demonstration project. The project staff are working tirelessly to process massive amounts of complex data.”
The successful demonstration of blasting in the bay using a bubble curtain blast attenuation system may now be used on the other piers – which are all that remains of the Bay Bridge after construction of a new Skyway between San Francisco and Oakland.
This is taken from the January-February 2016 issue of Demolition & Recycling International. To see the full article, including additional images, or to receive the magazine on a regular basis, please visit www.khl.com/subscriptions